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3.1 Ideal cycle description Chapter 3: Ideal Cycle Considerations A typical subcritical ideal vapor compression cycle is shown in Figure 3.1. Evaporation takes place at a constant temperature and pressure, with exit quality equal to one. At the entrance to the compressor, the enthalpy (h1) and entropy (s1) are calculated from the saturation properties of the refrigerant based on the evaporating temperature. The exit enthalpy from the compressor (h2) is calculated based on isentropic compression from 1 to 2 with a compressor efficiency of one. Condensation is assumed to be isobaric, and the condenser exit quality is zero. From saturation properties of the refrigerant the exit enthalpy (h3) can be calculated based on the condensation pressure and exit quality or temperature. As a result, the thermodynamic cycle can be completely described for a specified evaporating temperature and exit temperature from the evaporator. h3 h4 Figure 3.1 Ideal subcritical cycle h2 h1, s1 Enthalpy If the discharge pressure is higher than the critical pressure of the refrigerant then the cycle is transcritical. In the ideal transcritical cycle, the refrigerant can be cooled at constant pressure from a supercritical vapor to a saturated liquid without passing through a two phase region of condensation at constant temperature. The critical temperature and pressure is 31.1oC and 7380 kPa for R744, and 72.1oC and 4925 kPa for R410A. Since the enthalpy at each state point is specified, the capacity of the system is a function only of the mass flow rate of refrigerant supplied by the compressor. The ideal system’s heating and cooling capacities are: QH = m& refrigerant ⋅ (h2 − h3) (3.1) and, QC =m&refrigerant⋅(h1−h3) (3.2) Respectively, the ideal cycle coefficient of performance, COPc and COPh for heating and cooling can be calculated as: 12 TemperaturePDF Image | Comparison of R744 and R410A
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